Depot ships provide services unavailable from local naval base shore facilities. Industrialized countries may build naval bases with extensive workshops, warehouses, barracks, and medical and recreation facilities. Depot ships operating within such bases may provide little more than command staff offices,[1] while depot ships operating at remote bases may perform unusually diverse support functions. Some United States Navy submarine depot ships operating in the Pacific during World War II included sailors with Construction Battalion ratings to clear recreational sites and assemble buildings ashore,[2] while the Royal Navy mobile naval bases included specialized amenities ships to meet recreational needs of British Pacific Fleet personnel.[3]

Services provided by a depot ship depend upon whether typical client warship missions are measured in hours, or days, or weeks. A warship crew may be expected to remain at their stations for missions measured in hours, but longer missions may require provisions for dining, sleeping, and personal hygiene. The crew of small warships may carry individual combat rations and urinate or defecate from the weather deck. Longer missions typically require storage provisions for drinking water and preserved food, and some resting area for the crew, although rest may be limited to a sheltered spot to sit or recline. Cooking may be limited to warming food on an exhaust vent, and buckets may be used for bathing, laundry, and sanitary waste. Habitability standards vary among navies, but client warships large enough to include a head, bunks, a shower, a kitchen stove, refrigerated food storage, a drinking water distillation unit, and a laundry require little more than medical and repair service from a depot ship. Depot ships are similar to repair ships, but provide a wider range of services to a smaller portion of the fleet. Depot ships undertake repair work for a flotilla of small warships, while repair ships offer more comprehensive repair capability for a larger variety of fleet warships. Depot ships also provide personnel and resupply services for their flotilla. Some depot ships may transport their short-range landing or attack craft from home ports to launch near the scene of battle.[4] The following summary of World War II depot ships indicates the range of locations and warships served:

Nettlebeck, Brommy and Van der Groeben were depot ships for the 1st, 2nd and 3rd R boat flotillas, respectively. The 1st and 3rd flotillas were at Kiel, and the 2nd was at Cuxhaven.[13] HMS Ambitious (F169), Celebrity and St. Tudno served as depot ships for minesweepers. Ambitious was stationed at Scapa Flow, and St. Tudno was at the Nore.[5] Japan requisitioned Chohei Maru, Rokusan Maru and Teishu Maru from civilian service as depot ships for minesweepers.[14]

Tsingtau and Tanga were depot ships for the 1st and 2nd E-boat flotillas at Kiel and Hamburg, respectively.[13]Kamikaze Maru, Nihonkai Maru, Shinsho Maru and Shuri Maru were requisitioned from civilian service as depot ships for Japanese Motor Torpedo Boats.[14]

Some depot ships support a naval base. HMAS Platypus was the base ship at Darwin, Australia during World War II.[8] In the Royal Navy, under section 67 of the Naval Discipline Act 1866, the provisions of the act only applied to officers and men of the Royal Navy borne on the books of a warship. When shore establishments began to become more common it was necessary to allocate the title of the establishment to an actual vessel which became the nominal depot ship for the men allocated to the establishment and thus ensured they were subject to the provisions of the Act.[36]